Mass Spectrometry is the tool of choice to identify and quantify proteins in complex mixtures. Studying proteins by mass spectrometry helps us to understand how proteins function together to make a cell, how they endow cells with the ability to organize into tissues of remarkable complexity, such as the brain, and how aberrations in protein function can lead to diseases such as cancer. Proteins do not act in isolation, but rather exert their effects following assembly into multi-protein machines, or more evanescent protein complexes. We specialize in affinity-based proteomics to define these protein complexes and the identification of post-translational modifications contained within these complexes.